(Reuters) - Oracle Corp CEO Larry Ellison testified that SAP's theft of its software cost his company $4 billion, doubling his previous estimate but failing to cite written evidence of it.

SAP lawyers immediately challenged Ellison to back up the new figure, but Silicon Valley's richest man said he could not point to documentation proving his claim of the cost of lost business. Germany's SAP asserts that it owes Oracle merely tens of millions of dollars.

Oracle's star witness was unusually subdued in court on Monday, a departure from his sometimes heated public rhetoric.

He also failed to produce testimony of his public assertion that Hewlett-Packard Co's new CEO, Leo Apotheker, knew of the theft when he headed SAP and did nothing about it until Oracle sued.

A source told Reuters that Oracle had hired private detectives to locate Apotheker so he could be subpoenaed to appear in the trial.

The normally outspoken Ellison left the courtroom on Monday without speaking to reporters.

Oracle's lead trial attorney, David Boies, said outside court it was important for the jury to hear Apotheker in person, and thus it was "appropriate" to try to locate the HP CEO. Boies declined to discuss specifics of Oracle's search.

"The more detail we go into publicly, the less chance we have to find him," he said.

Ellison's highly anticipated testimony on Monday fell short of his brash, accusatory statements against Apotheker and SAP in recent weeks, in both tone and substance.

But JMP Securities analyst Pat Walravens said Ellison succeeded in his most important task in court.

"What they accomplished was to get that $4 billion figure out there to the jury from a credible source," he said. "Ellison was the best chance to do that."

Walravens said it is too soon to say whether Oracle is winning, as SAP has not yet begun to present its side. Oracle is expected to rest its case this week.